All known Rotating Biological Contactors (RBCs) have a common rotating shaft surrounded by media. These secondary biological treatment devices are exposed to the primary clarifier effluent and constantly rotate during the treatment process. This rotational force causes the solids to accumulate in the shaft area.
Applicant is aware of the following U.S. patents which revolve around rotating biological contactors: U.S. Pat. No. 232,236 to LaBorde for a module for a filter media or similar article; U.S. Pat. No. 232,237 to LaBorde for a module for a filter media or similar article; U.S. Pat. No. 232,238 to LaBorde for a module for a filter media or similar article; U.S. Pat. No. 2,212,932 to Fairlie for a filling material for reaction spaces; U.S. Pat. No. 3,364,656 to Whiton, et al for a method for effecting countercurrent contacting of gas and liquid streams; U.S. Pat. No. 3,540,589 to Boris for an apparatus for the purification of polluted water; U.S. Pat. No. 3,758,087 to Hoon, Jr. for a contact device; U.S. Pat. No. 3,882,027 to Lunt for a random packing medium; U.S. Pat. No. 3,913,890 to Lankenau, et al for relatively hard unattached objects to inhibit caking and scaling in a horizontal pipeline; U.S. Pat. No. 3,957,931 to Ellis, et al for a fluid-fluid contact method and apparatus; U.S. Pat. No. 4,115,269 to Bennett, et al for random packing materials; U.S. Pat. No. 4,137,172 to Sako, et al for rotating biological contactor; U.S. Pat. No. 4,160,736 to Prosser for a rotating trickling filter; U.S. Pat. No. 4,200,532 to Iwatani, et al for a waste water treatment apparatus; U.S. Pat. No. 4,333,893 to Clyde for a high area contactor; U.S. Pat. No. 4,425,285 to Shimoi, et al for a packing material unit; U.S. Pat. No. 4,554,114 to Glen, et al for a packing element and method using same; U.S. Pat. No. 4,668,442 to Lang for column packing; U.S. Pat. No. 4,842,920 to Banai, et al for plastics elements for inordinate film-flow packings; U.S. Pat. No. 4,921,641 to Lavin for liquid-vapor contact columns; and, U.S. Pat. No. 5,112,760 to Baumgartner, et al for a mass transfer membrane for oxygenation of animal cell reactors. Applicant is also aware of the following foreign patents: British Patent No. 1,275,116 and French Patent No. 1,405,374.
Rotating Biological Contactor (RBC) technology involves a fixed film, attached growth biological reactor for reducing the organic components of domestic and industrial waste waters.
RBCs consist of a center shaft, usually constructed of steel, with plastic sheets (Media) attached to that shaft. These sheets of media are typically twelve feet in diameter. The media is typically forty percent (40%) immersed in waste water. As the shaft rotates, films of the waste water adhere to the plastic media. Bacteria, present in the waste water, adhere to the plastic sheets as a film of slime. This film is called the bio-mass. While it is impossible to count each bacteria or the clusters of bacteria, the population of bacteria is determined by the square footage of surface area that the plastic media provides as residence.
Bacteria, like people, require two basic fundamental elements to sustain their life and propagate: 1. Oxygen and 2. Food. As the RBC rotates, the bacteria are subjected to the atmosphere, which is their source of oxygen. As the rotation continues, the bacteria become immersed in the waste water. The organic constituents of the waste water become the food source for the bacteria. Thus, as the bacteria sustain their own life, they reduce the contamination in the waste water by utilizing the organic matter as food.
This method of waste water treatment was developed in the early 1970's. There was some activity in Europe in 1968, but these were really preliminary efforts that led to the current technology. The earliest studies done in the United States on prototype units were done in about 1973. The RBC process employs four basic components in its approach. There is a concrete holding tank, a central shaft to which is attached the "media" the "Media" itself and a power transmission system that keeps the media in constant rotational motion.
The tank provides the environment in which the process occurs. It constricts the path of the waste water stream to allow maximum exposure of the waste stream to the "media". The steel shaft can consist of a hollow tube or solid shaft and can be round, square, or octagonal in shape.
The "media" is the heart of the process. The "media" provides the surface area for bacteria to attach themselves. The greater the surface area available, the larger the bacteria population for the digestive process. In conventional RBCs, the media consists of alternate layers of formed and flat sheets of polyethylene, thermally welded to produce controlled, uniform spacing. The media is attached to the shaft through a hub assembly and the final shape of the RBC is a cylindrical drum.
In the actual operation of the RBC unit, the polyethylene sheet is submerged forty percent into the waste water stream. By a constant rotational action (approximately 1.6 revolutions per minute) the media surfaces are kept in contact with the waste stream. This rotational action is designed to alternately expose the bacteria to the waste stream as a food source and to then expose the bacteria to the air for oxygenation. It is through this alternate exposure that the digestive process is optimized.
The major advantage of this process is that it is simple and energy efficient to run. The installation is cost competitive with other systems; however, the major cost advantage is in its low energy requirements during operation. The biological effectiveness of the process makes it attractive; however, the current technology does not optimize bacteria growth, nor utilization of the bio-mass. Disadvantages of RBCs are numerous. From a process performance perspective, RBCs have a limited oxygenation capacity. The media is only forty percent immersed in the waste water. Therefore, a "dead zone" exists near the center shaft. Solids accumulate and build-up near the center shaft because radial velocities are minimal, and the area is subject to minimal water scouring. Because of these reasons, many RBCs never perform to the standards of the original design. Operational data demonstrates the system is unreliable, inconsistent, and undependable.
The media has experienced problems of collapsing due to the weight of the solids build-up and retention. Additionally, many problems have been realized in the failure of the bonding of the plastic media sheets to the center steel shaft.
Shaft problems have been experienced where the shaft would shear under the load of excessive solids retained by the media. Because of this anticipated excessive loading, bearing problems have also been realized.